Which ones? I have it turned off and haven't had any issues with games.

Games that use Peer-to-peer lobbies instead of dedicated servers, more popular with multiplayer co-op games.

Typically, it can be possible to join another lobby, but impossible to host (insofar as other people can't connect to it)

Lets not forget about consoles too. Xbox Live and PSN complain about obstructive NAT configurations and rely upon uPnP to open ports.

Of course they can be opened manually but that assumes some technical experience, and that the ISP provided hardware gives you access to its configuration.

This is also a problem with games using the client/server model where one of the players is the server.

Ubisoft games come to mind. Without UPnP or specific ports forwarded you’ll have limited NAT support which many games will tell you.

Yeah, you can’t really “manage your firewall” when consumer software doesn’t open fixed ports and assumes upnp.

Why not? Which consumer software breaks?

Normally people say games. I have disabled upnp on my firewall and there're two gaming PCs, a PS3, a PS4 and a PS5 running happily behind it. I just finished a Demon Soul's session with voice chat with friends with no problems. NAT type 2, because I managed my firewall to enable this.

Have you tried a client/server style game not relying on Steam multiplayer?

It makes no difference since as I wrote I manage the firewall to allow this. But yes, since none of the games on consoles use Steam. If NAT wasn't set up I would get NAT type 3 on the PlayStations for example.

ETA: My point was to get an example of something that breaks "because it doesn't work without upnp". I have yet to see a game that doesn't support a fixed set of ports.